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BIAS MAGNETRON SPUTTERING FOR NIOBIUM THIN FILMS. A.Frigo, G.Lanza ,A.Minarello H.Padamsee, V.Palmieri Università degli Studi di Padova Istituto Nazionale di Fisica Nucleare Cornell University. The International Workshop on

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bias magnetron sputtering for niobium thin films

BIAS MAGNETRON SPUTTERING FOR NIOBIUM THIN FILMS

A.Frigo, G.Lanza,A.Minarello

H.Padamsee, V.Palmieri

Università degli Studi di Padova

Istituto Nazionale di Fisica Nucleare

Cornell University

The International Workshop on

THIN FILMS AND NEW IDEAS FOR PUSHING THE LIMITS OF RF SUPERCONDUCTIVITY

slide2

Bias Magnetron Sputtering for Niobium thin films

  • Advantages and disadvanteges of the bias tecnique
  • Preliminary results of a mixed bias-magnetron sputtering configuration for coating Niobium on copper 1.5 GHz cavities
  • First applications of a large area cavity shaped cathode in the bias diode sputtering configuration.
slide3

Bias Sputtering

The positive bias applyed to the grid between target and substrates promotes IONIC BOMBARDMENT OF THE GROWING FILM

-

Target

+

Biased Grid

Substrate

slide4

Diode Bias Sputtering

IONIC BOMBARDMENT OF THE GROWING FILM

slide5

Diode Bias Sputtering

Impurities re-sputtering

during the film growth

slide6

Diode Bias Sputtering

Impurities are preferentially removed relative to the atoms of the main film.

fraction of impurities trapped into the film

i = impurities sticking coefficient

Ni = atoms impurities arriving on the film

β= function of the bias current due to impurities ions

R= sputtering rate

L.I.Maissel, P.M.Schaible; J.Appl.Phys. 36, 237 (1965)

slide7

Advantages

  • Densification of the crystal structure
  • Higher sputtering rate
  • Lattice rearrangement
  • Films quality improvement
slide8

Advantages

  • Increasing of the coating hardness
  • Similar defect annealing as does an elevated substrate temperature (E.Kay,G.Heim;J.Appl.Phys 49 (9) 4862 (1978))
  • Electrons bombardment reduction
  • Adhesion improvement
slide9

Disadvantages

  • Noble gas atoms embedding
  • Lattice defects
  • Thickness reduction
  • Biased grid shadowing
  • Still hydrogen removal is low
slide10

Bias Sputtering

500Å

1000Å

5000Å

Ta Resistivity (microhom-cm)

High Resistivity Cathode

Ta Resistivity (microhom-cm)

Low Resistivity Cathode

Substrate Bias (Volts)

Substrate Bias (Volts)

High bias voltage reduce differences between films sputtered from different cathodes and of different thickness. (Tantalum films studies-L.I.Maissel,P.M.Schaible,J.Appl.Phys. 36,237 (1965) )

slide11

The Niobium case

Ta Resistivity (microhom-cm)

Temperature coefficient of resistance (x10-3)

Negative Bias Potential (Volts)

Electrical resistivity and temperature coefficient of resistance of niobium films deposited on negatively biased substrates as a function of bias potential. ( J.Sosniak,J.Appl.Phys. 39,4157 (1968) )

slide12

The Niobium case

Ic

R

Ib

Film Deposition Rate Å/min

Current (milliamperes)

Negative Bias Potential (Volts)

Deposition rate increases with increasing negative bias. (J.Sosniak,J.Appl.Phys. 39,4157 (1968) )

slide14

Standard CERN coating configurations

Magnet

Cylindrical

Magnetron

Cavity

Niobium cathode

slide15

Standard CERN coating configurations

Cooling air

Ceramic insulator

Niobium cathode

- 450 V

Stainless steel vacuum chamber with cavity shaped sample holders

Moving magnet

Niobium sputtered atoms

Glow discharge

Argon

entrance

To the vacuum pumps

slide17

Biased Grid +100 V

INFN-LNL coating configuration

Cathode - 250 V

Magnet

Grounded Cavity

slide18

Second Improvement

Combination of the CERN coating configuration and the bias sputtering technique made from INFN-LNL

-

S

N

S

Magnets

S

N

N

Target

+

Biased Grid

Substrate

slide20

Water out

Biased Magnetron Sputtering:the construction

Improvement of the cooling system

Water in

slide22

BIAS

CERN type

Biased Magnetron Sputtering: RRR results

The grid still doesn’t affect much the equator part

slide23

BIAS

CERN type

Biased Magnetron Sputtering: thickness

Sputtering rate obtained from thickness measurement

slide24

Biased Magnetron Sputtering: Tc results

BIAS

All samples with RRR>8 show a Tc higher than 9,3 K

slide25

Biased Magnetron Sputtering: lattice results

BIAS

Film show a lattice parameter lower than the Nb bulk

They are grown with compressive stress

slide27

-

Target

+

Biased Grid

INFN-LNL coating configuration II

The grid is behind the cathode

Substrate

slide28

INFN-LNL coating configuration II

The grid is behind the cathode

  • Advantages:
  • Anode-cathode distance reduction
  • Higher cathodic area
  • No shadowing due to the grid
slide29

A

B

Substrate

Cathode

BIAS

INFN-LNL coating configuration II

Plasma is conductive

The bias grid can be placed behind the cathode

slide30

Bias Sputtering

Bias CERN

Low ratio cathode/substrate area

Low sputtering rate (1 micron /day)

slide31

Cavity Shaped Cathode

High ratio cathode/substrate area

slide33

Cavity Shaped Cathode

Biased stainless steel tube

Cathode -300 V

Grounded Cavity

Insulator

slide34

Cavity Shaped Cathode

Vc = -300 V

i = 5 A

p = 6x10-2mbar

slide35

Summary

  • Mixed Bias Magnetron Sputtering
    • preliminary results (RRR, Tc, lattice)
    • studies with different bias and parameters
    • studies with shaped grid
    • test the cavity
  • Large Area Cavity Shaped Cathode
    • construction and first run
    • improvement of the structure stability
    • characterization of the films
    • test the cavity
slide38

INFN-LNL coating configuration

Biased stainless steel tube

Cathode -300 V

Grounded Cavity

Insulator

slide39

Cavity Shaped Cathode

V=250 V

i=8 A

p=1x10-2mbar

10 cm

60 G